Gramfeed is a side-project turned startup that grew quite successfully with the rise of Instagram.
At launch it was just a website to view popular photos, search photos, view user photos and view user feed.
Most of the features and UI changes were implemented based on user feedback … Obviously watch for any product mentions on twitter and engage with users.
The key is to select a few bloggers and tell them your personal story that led you to developing the website rather than describing how great your product is.
Ultimately if your product is simple, easy to use and solves a problem then people will notice it, they will tweet about it, recommend it to their friends, influencers will take notice and you will get covered eventually, no special marketing is necessary, focus on making things simple and listen to your customers.
Having a blog also drives traffic back to the site.